In wireless communications, a communication system usually employs a transmitter to send transmitted signals in the form of an electromagnetic wave to a receiver through a physical medium such as air. Typically, the received signal at the receiver contains distortions due to imperfect channel effects such as multipath reflection and fading effects. Orthogonal Frequency Division Multiplexing (OFDM) is an effective telecommunication technology to deal with the multipath reflection issue. In the OFDM system, the receiver only requires a simple one-tap equalizer to equalize the frequency-selective fading effect in the received signal caused by the multipath. As a consequence, the OFDM technology has become popular among all wired or wireless communication systems and digital broadcast applications, including Asymmetric Digital Subscriber Line (ADSL) systems, Power Line Communication (PLC) systems, Digital Audio Broadcasting (DAB) systems, wireless Local Area Network (WLAN) 802.11a/b/g/n systems, Digital TV Standards such as China Mobile Multimedia Broadcasting (CMMB), Digital Video Broadcasting-Terrestrial (DVB-T), and Digital Video Broadcasting-Handheld (DVB-H), Wi-Max IEEE 802.16e equipped with mobility equipment, and so on. Furthermore, fourth-generation (4G) wireless communication standards such as IEEE 802.16m and 3GPP (3rd Generation Partnership Project) Long Term Evolution-Advance (LTE-A) also employ the OFDM transmission technology.
High-speed motion is a feature adopted in 4G wireless communication standards such as the 802.16m and the 3GPP LTE-A. The device is capable of handling communication requirements under a high-speed environment with a motion speed as fast as 350 km/hr, even up to 500 km/hr. High-speed motion enabled communication devices have found application in broadband wireless telecommunication in a high-speed rail system. Rather than working in a static condition, the receiver in the OFDM system now moves at a high speed relative to the transmitter, consequently each useful OFDM symbol duration in the communication channel is no longer a fixed value, rendering a time-selective fading channel. Under high-speed motion, the Doppler effect causes the carrier frequency of an OFDM carrier to shift plus or minus 1 time of the Doppler frequency (fd). The Doppler shift in the OFDM system may render an Inter-Carrier Interference (ICI) effect between sub-carriers in the received signal at the receiver, destroy the orthogonality and devastate system performance, resulting in an error floor effect.
The ICI effect may be reconstructed by a certain linear combination of transmitted data and channel variation response in a frequency domain. The more accurate the channel estimation is, including a channel average response and variation response, the more accurate the detected transmission data and the reconstruction of the ICI effect becomes, leading to a reduced ICI effect and increased performance of the OFDM system in the high-speed environment. However, the main challenge for channel estimation of the high-speed motion lies with the known signal, such as when the pilot sub-carrier signal has become distorted due to the ICI effect. Also, since the channel environments are different from the previous and subsequent symbol times, the traditional averaging technique in time domain for enhancing the accuracy of the channel average response is no longer applicable in high-speed conditions.